Sintering Behavior and Mineralization Mechanism of Red Mud for Pyrometallurgical Iron Extraction

Abstract

To address the resource waste caused by the ineffective recycling of large quantities of red mud, this study proposes an innovative technical route consisting of red mud sintering followed by smelting in a small blast furnace or solid-waste smelting furnace for pyrometallurgical iron extraction. Under optimized process conditions—binary basicity of 4.97, a raw material composition of 78.27% wet-based red mud, 15.67% quicklime, and 6.05% fuel, with a solid fuel consumption of 121 kg/t—the produced sinter meets the feeding requirements of blast furnace smelting. The results indicate that the liquid phase generated during red mud sintering mainly consists of composite oxides in the CaO–Al₂O₃–SiO₂ system; calcium aluminosilicate (Ca₂Al₂SiO₇) was detected and inferred to be a potential bonding phase in the sinter matrix. Thermodynamic analysis shows that the Gibbs free energy of Ca₂Al₂SiO₇ is lower than that of calcium ferrite, indicating that its formation is thermodynamically more favorable. The formation amount of this phase is closely related to the Ca/Al ratio, while temperature has a limited influence. In addition, Na₂O can react with CaO·2 Al₂O₃ to form a low-melting-point phase, which significantly reduces the sintering temperature and enhances the fluidity of the liquid phase. These findings provide a new theoretical basis for the sintering of high-alumina ores and offer technical support for the efficient utilization of red mud as well as energy conservation and emission reduction.

1. Introduction

Red mud is a solid waste generated during the production of alumina, with approximately 1–2 tons of red mud produced for every ton of alumina manufactured [1,2]. According to statistics, China’s alumina output reached 67.831 million tons in 2024, accounting for approximately 60% of the global production. In the same year, about 115 million tons of red mud were newly generated, while the cumulative stockpile exceeded 1.5 billion tons. However, the comprehensive utilization rate of red mud remains only about 12%, which is far behind the rapid development of the alumina industry. Red mud mainly contains Fe₂O₃, Al₂O₃, SiO₂, CaO and Na₂O [3,4,5,6], and exhibits strong alkalinity. Large-scale stockpiling of red mud not only leads to significant resource waste and land occupation, but also poses potential risks of groundwater pollution.

At present, the utilization approaches of red mud mainly include cement production [7,8,9], construction brick manufacturing [10,11,12], preparation of thermal insulation rock wool products [13], flue gas desulfurization [14], soil improvement [1], and adsorption of fluoride and lead ions from wastewater [15,16,17,18]. However, many of these applications still carry the potential risk of secondary pollution. In terms of resource recovery, various methods have been explored worldwide, including magnetic separation [19,20,21,22,23,24], wet recovery processes [25], application as a dephosphorization agent in steelmaking, co-sintering for iron and aluminum recovery [26], and direct iron reduction in electric furnaces under an Ar–10% H₂ atmosphere [27]. Nevertheless, these processes generally involve high costs and face difficulties in large-scale industrial application.

Approximately 95% of the global alumina is produced by the Bayer process, and the red mud generated from this process typically contains a high Fe₂O₃ content. After magnetic separation, the total iron content can exceed 45%, which meets the requirements for low-grade sintering raw materials. Therefore, red mud can potentially be utilized as a component of sintering feed. The iron can subsequently be recovered through smelting in a small blast furnace, specifically for the integrated process route of red mud valorization via sintering followed by iron recovery, while the final slag can be used as premelted slag or synthetic slag. Compared with existing recovery methods, this process route exhibits several advantages, including a short process flow, low production cost, simultaneous recovery of iron and aluminum, high red mud consumption capacity, and minimal risk of secondary pollution. Compared to the traditional sintering process in the iron ore industry, using red mud sintering for iron recovery presents substantial economic and environmental advantages.

However, the main challenge of this process lies in the high Al₂O₃ content in red mud. When the Al₂O₃ content in iron ore exceeds approximately 3%, the sintering difficulty increases significantly. Al₂O₃ reacts with Fe₂O₃ to form high-melting-point aluminoferrite phases (melting point around 1600 °C), which inhibit the formation of calcium ferrite (SFCA). As a result, the sintering temperature window becomes narrower and the formation of the liquid phase is delayed. Consequently, the reduction degradation index (RDI) and tumbler index (TI) of sintered ore decrease significantly. In addition, the crack sensitivity of sintered ore increases, and its compressive strength may decrease by about 15–20%, seriously affecting the quality of the sintered product. Meanwhile, the viscosity of the liquid phase also increases [28]. When the Al₂O₃ content in red mud exceeds 11%, it becomes necessary to explore suitable sintering routes under high-Al₂O₃ conditions and to clarify the sintering mechanism in a high-aluminum environment. In addition, Na₂O contained in red mud can act as a flux, forming low-melting-point silicate phases that promote particle bonding and facilitate the sintering process. Therefore, the influence of Na₂O on the sintering temperature and liquid phase formation was also analyzed in this study.

To address these challenges, a large number of simulation calculations and laboratory experiments were conducted to explore the sintering process route of red mud under high basicity conditions. The sintering mechanism of red mud was systematically investigated, providing a new theoretical basis for the sintering utilization of high-aluminum iron ores.

2. Experimental

The chemical compositions of the raw materials used in this study, including red mud, limestone, and coke breeze, are presented in

Table 1. Chemical composition of red mud (wt%).

Compositions	TFe	CaO	SiO2	MgO	Al2O3	TiO2	MnO	Na2O	H2O
Wt%	48.31	1.64	3.07	0.28	10.84	3.56	0.10	0.862	9.74

,

Table 2. Chemical composition of limestone (wt%).

Compositions	CaO	SiO2	MgO	Al2O3	Fe2O3	H2O
Wt%	86.03	2.16	4.95	0.65	0.91	-

and

Table 3. Chemical composition of coke breeze (wt%).

Compositions	Fixed Carbon	Ash	Volatile Matter	S	CaO	SiO2	MgO	Al2O3
Wt%	84.83	14.15	1.02	1.04	-	-	-	-

.

Red mud, the primary feedstock, was obtained from an alumina refinery in Hebei, China, and is a typical byproduct of the Bayer process for alumina production. Its chemical composition is detailed in Table 1.

Limestone, employed as a fluxing agent, was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), with its composition provided in Table 2.

Coke breeze, used as both fuel and reductant in the sintering process, was obtained from a local steel plant in Hebei, China. Its proximate analysis and ash composition are summarized in Table 3.

All materials were characterized by standard chemical analysis methods to ensure the quality and stability of the resulting sinter. Based on the chemical assay data, the proportions of raw materials were precisely calculated using the trial-and-error method and single-ore method, which enabled the composition of the sinter to be kept stable, with the fluctuation of the fuel ratio controlled within ±0.2%.

Figure 1 illustrates the technical route of this study, presenting the complete experimental procedure from material preparation to mechanistic analysis. The closed-loop optimization logic provides support for the reliability of process parameters and subsequent mechanism analysis.

2.1. Sintering Process Parameters

Because red mud contains a large amount of alumina, it increases the sintering temperature. According to simulation results from Factsage and preliminary experimental data, a sufficient liquid phase ratio for sintering can only be achieved at low temperatures under high basicity conditions.

The basicity parameters were controlled at R₂ = 4.97 and R₅ = 1.15, and the mass fraction of ferrous iron was maintained at 12 ± 3%. The mass fractions of wet-based red mud, limestone, and fuel were 78.27%, 15.67%, and 6.05%, respectively. The corresponding solid fuel consumption was 121 kg/t.

All simulations were performed in triplicate to ensure convergence, and experimental trials were repeated at least three times to confirm reproducibility of the results.

2.2. Determination of Tumbler Index

The determination of the sinter drum index is significant for assessing the mechanical strength of sintered ore, ensuring its excellent resistance to breakage and abrasion during the blast furnace smelting process. Core parameters: drum diameter: 1000 ± 5 mm, drum length: 500 ± 5 mm, rotational speed: 25 ± 1.5 r/min, sample weight: 15 kg, preset revolutions: 200 rev, motor power: 1.5 kW.

The sample was placed in a tumbler drum and rotated at a speed of 25 r/min for 200 revolutions. After rotation, the material was sieved, and the mass fraction of particles with a size larger than 6.3 mm was defined as the tumbler index (TI).

The standard GB/T 14201-2018 “Test method for determining the compressive strength of iron ore agglomerates using a tumbling drum” specifies that the feed sample size shall be sintered ore particles within the range of 10–16 mm, with fines and oversized particles removed to ensure uniform particle size distribution [29]. Each experimental condition was replicated three times.

2.3. Determination of Reducibility

The Reducibility Index (RI) refers to the ease with which oxygen bound to iron in sintered ore is captured by gaseous reducing agents (CO) under simulated reduction conditions in the upper blast furnace (900 °C, CO/N₂ atmosphere), expressed as the oxygen removal rate per unit time. A higher RI indicates that the ore is more susceptible to indirect reduction, which is more conducive to reducing the coke ratio in the blast furnace.

A 500 g dried sample with a particle size of 10–12.5 mm was weighed and placed in a fixed-bed reduction apparatus. The sample was reduced at a constant temperature of 900 °C for 180 min. The reducing gas flow rate was 15 L/min, with a composition of 30% CO and 70% N₂.

The standard is ISO 7215:2015 “Iron ore—Determination of reducibility”; The temperature control accuracy is ±5 °C; the weighing precision reaches 0.01 g; the gas flow is computer-controlled [30]. The system records the time-mass loss curve for reduction behavior analysis. Each experimental condition was replicated three times.

2.4. Determination of Softening–Melting–Dripping Properties

The determination of softening and melt-dripping properties is primarily used to evaluate the physical state changes of materials during heating and their structural stability under high-temperature conditions.

The main experimental setup consisted of a melting–dripping furnace, a gas distribution system, and a computer-based data acquisition and processing system. The melting–dripping furnace was the core equipment and included a high-temperature heating furnace, a crucible device, an intelligent temperature control unit, a loading system, a real-time pressure difference recording system, and a material layer displacement measurement system.

The graphite reactor had dimensions of Φ75 × 200 mm. The heating procedure was as follows: the temperature was increased from 0 to 900 °C at a rate of 10 °C/min, maintained at 900 °C for 60 min, and then increased above 900 °C at a rate of 5 °C/min. All samples had a particle size of 10–12.5 mm. When the sample temperature reached 500 °C, the reducing gas was introduced at a flow rate of 15 L/min, with a composition of 30% CO and 70% N₂.

The standard is GB/T 34211-2017 “Determination Method of Softening and Melting Characteristics of Iron Ore” [31]. Each experimental condition was replicated three times.

2.5. Determination of Phase Composition and Element Distribution of Sintered Ore

The phase composition of the sintered ore was analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD was performed on a Bruker D8 ADVANCE diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with a Co target, operating at a tube voltage of 35 kV and a tube current of 40 mA. The scanning range was set from 10° to 90° 2θ with a step size of 0.02° and a scanning speed of 2°/min. A Lynxeye XE detector was used for high-sensitivity data collection. SEM observations and elemental analysis were conducted using a ZEISS GeminiSEM 300 scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany) coupled with an Oxford Ultim Max EDS system (Oxford Instruments PLC, Abingdon, UK).

Sintered ore samples were crushed and sieved to a particle size of 10–12.5 mm. For XRD analysis, representative subsamples were ground to a particle size of <0.074 mm and homogenized. For SEM observation, polished cross-sections were prepared by mounting samples in epoxy resin, followed by grinding with SiC papers and polishing with diamond pastes to a mirror finish. Carbon coating was applied to non-conductive areas to avoid charging effects during imaging.

2.6. Thermodynamic Simulation

FactSage software (v.8.2) was used to simulate and calculate the liquid phase composition and solid phase distribution during the red mud sintering process. The FToxid database was selected for thermodynamic calculations. The Phase Diagram module was used to construct isobaric phase diagrams of the relevant system and to analyze the stable phases under different temperatures and compositions.

The Equilib module was employed to calculate the equilibrium phase composition of the system at atmospheric pressure over a temperature range of 800–1500 °C. The liquid phase fraction, the major liquid and solid phase components at different basicity levels, as well as mineral phase evolution, melting temperature curves, Gibbs free energy, and phase diagrams were obtained. These results were used to clarify the sintering behavior and mineralization formation mechanism of red mud.

2.7. Determination of Melting Point

The melting point was measured by a fully automatic melting point apparatus with argon protection throughout the test. The sample was loaded into a crucible, and the linear heating rate was controlled at 5 °C/min. The morphological changes of the sample were monitored in real time by the built-in high-definition optical imaging system, and the temperatures at different shrinkage rates were recorded synchronously. Deformation temperature (DT, 10% shrinkage), softening temperature (ST, 25% shrinkage), hemispherical temperature (HT, 50% shrinkage) and flow temperature (FT, 75% shrinkage) were obtained, and the flow temperature was selected as the final melting temperature.

2.8. Determination of Viscosity

The viscosity was measured using a high-precision automatic rotational viscometer. The sample was slowly poured into the sample cup, and the entire test was conducted under an argon atmosphere. The heating rate was set to 3°C/min, and the rotor rotational speed was fixed at 60 r/min before starting the test procedure. The instrument continuously monitored and recorded the viscosity at various temperatures in real time. When the viscosity became thermally stable and the data fluctuation was less than 2% over three consecutive tests, the corresponding viscosity value at this temperature was recorded.

3. Results

3.1. Chemical Composition of Sintered Ore

Chemical compositions of the sintered ore obtained after sintering are presented in

Table 4. Chemical composition of sintered ore (wt%).

Compositions	TFe	FeO	CaO	SiO2	MgO	Al2O3	TiO2	Na2O
Wt%	43.20	14.79	18.57	4.69	1.36	9.59	2.64	0.62

. As shown in Table 4, the mass fraction of red mud in the sinter mixture was 48.31%. The measured iron content was close to the calculated value, and the FeO content was 14.97%. In addition, the contents of CaO and Al₂O₃ in the sintered ore were relatively high, which was consistent with the theoretical calculations based on the raw material proportioning. According to the data in Table 4, both the TFe and FeO contents of the sintered ore met the application requirements for small blast furnace operation.

3.2. Tumbler Index Results of Sintered Ore

The results of the drum test are presented in

Table 5. Tumbler index test results of sintered ore (wt%).

Tumbler Index %	>40 mm	40–25 mm	25–16 mm	16–10 mm	10–5 mm	<5 mm
75.06	5.08	22.14	48.48	17.55	6.21	0.54
74.13	5.74	39.11	34.48	13.54	6.63	0.5

. According to the test results, the tumbler index (TI) of the red mud sinter reached approximately 75%, indicating satisfactory mechanical strength and meeting the requirements for smelting in small blast furnaces. The particle size distribution of the sinter was mainly within the range of 10–40 mm, accounting for about 88% of the total. Although the particle size tended to be relatively fine, the distribution was relatively uniform, which can satisfy the requirements for blast furnace operation.

Each sintering condition was tested in triplicate to ensure reproducibility, and the results presented are the average values, with standard deviations within ±3%.

3.3. Reducibility Results of Sintered Ore

The variation in reducibility and weight loss of the red mud sintered ore is presented in Figure 2. The reducibility index (RI) reached 86.17%, which was higher than that of conventional iron concentrate sintered ore.

3.4. Softening–Melting–Dripping Performance Results of Sintered Ore

The analysis results of the softening–melting–dripping behavior of the red mud sintered ore are presented in

Table 6. Softening–melting–dripping performance test results of sintered ore.

T10% (°C)	T40% (°C)	ΔT1 (°C)	Ts (°C)	ΔP (Kpa)	Td (°C)	ΔT2 (°C)	ΔH (mm)	S (kPa°C)
1083.1	1184.7	101.6	1181.2	30.00	1412.8	291.6	40.12	4536.73

Note: T10%: Initial softening temperature; T40%: 40% softening temperature; ΔT₁: Softening interval; Ts: Initial melting temperature; ΔP: Maximum pressure difference; Td: Initial dripping temperature; ΔT₂: Melting–dripping interval; ΔH: Material layer shrinkage; S: Comprehensive melting–dripping index.

. The softening temperature interval was 101.6°C; the melting temperature interval was 291.6°C and the melting–dripping interval was about 231.6 °C, both of which were wider than those of conventional sintered ore. In blast furnace smelting, an excessively wide softening–melting interval may lead to the thickening of the softening–melting zone and a reduction in the permeability of the burden layer. However, the obtained values still met the operational requirements for small blast furnace smelting. To address the potential permeability deterioration caused by the widened softening–melting zone, measures such as optimizing burden distribution, adjusting particle size composition to improve gas permeability, and fine-tuning blast furnace operating parameters are necessary to enhance the overall smelting performance.

Based on the above experimental results, the sintered ore prepared with red mud as the primary raw material met the industrial quality requirements of sintered ore and was suitable for smelting in small blast furnaces. However, when applied in large blast furnaces, the widened softening–melting zone may lead to poor permeability of the burden layer inside the furnace. The mineralization formation mechanism and influencing factors of red mud sintered ore are further analyzed and discussed in the following section.

3.5. Phase Analysis Results of Sintered Ore

The results of scanning electron microscope analysis are presented in Figure 3. The white phase in the image is Fe₂O₃, occurring as pure hematite grains and calcium ferrite. A certain amount of calcium aluminate bonding phase is present in the darker regions of the image. The hematite grains are consolidated through the combined action of calcium ferrite-based and calcium aluminate-based bonding phases.

The phase analysis results of the sintered ore are presented in Figure 4 The main iron-bearing phases were Fe₃O₄ and FeO, while the liquid phase mainly consisted of composite oxides formed by CaO, Al₂O₃, and SiO₂. The bonding phase in the sintered ore was mainly gehlenite (Ca₂Al₂SiO₇). When coexisting with other oxides such as SiO₂, CaO, or Na₂O, gehlenite can form low-melting-point eutectic mixtures. This bonding phase is significantly different from the calcium ferrite bonding phase commonly observed in conventional sintered ore.

4. Discussion

4.1. Influence of Mechanism on Phase Formation of Sintered Ore

4.1.1. Influence of Basicity on Phase Formation of Sintered Ore

Traditional sintering processes conventionally adopt low-basicity formulations to balance sinter reducibility and mechanical strength. However, red mud contains inherently high alumina content. Under low-basicity conditions, abundant alumina readily forms refractory aluminosilicate phases with elevated melting points, increasing sintering temperature and energy consumption while reducing process efficiency. Thermodynamic simulation results confirm that elevating sintering basicity represents an effective strategy to mitigate this challenge.

The liquid phase fraction, as well as the main components of the liquid and solid phases, was calculated using FactSage software for a system consisting of 100 g of red mud with added pure CaO, corresponding to basicity values of 2.6, 3.0, 3.5, 3.9, 4.4, and 4.8. The simulation results are presented in Figure 5. The results showed that the initial liquid phase formation temperature decreased with increasing basicity. When the basicity was 2.6, the initial liquid phase formation temperature was 1125 °C, and the liquid phase fraction reached 50% at 1275 °C. In contrast, when the basicity increased to 4.8, the liquid phase fraction reached 50% at only 1150 °C. These results indicate that under high-basicity conditions, a sufficient amount of liquid phase required for sintering can form in red mud at relatively lower temperatures, which is beneficial for the sintering process.

XRD qualitative analysis indicated the existence of calcium aluminosilicate in liquid-phase component. Since no quantitative characterization was performed, the primary constituent of the liquid phase cannot be determined to be calcium aluminosilicate.

Under high basicity conditions, CaO reacts with calcium aluminosilicate to form low-melting-point eutectic oxides, effectively reducing the sintering temperature. This explains why high basicity is indispensable for red mud sintering.

Calcium aluminosilicate-rich phases strengthen the sinter bonding matrix, improve tumbler strength, and develop an open crystal structure that enhances reducing gas diffusion, boosting reducibility. Concurrently, they improve liquid phase fluidity, lower viscosity in the softening–melting zone, and facilitate effective slag–metal separation. Collectively, these effects enable high-alumina sinters to meet metallurgical quality requirements.

4.1.2. Influence of Temperature on Phase Formation in Sintered Ore

The red mud sintering system is complicated with various coexisting mineral phases. Nevertheless, calcium aluminosilicate constitutes the dominant liquid-phase component, and calcium ferrite is the key iron-bearing product during sintering. Accordingly, these three typical core phases were selected for Gibbs free energy calculation, which simplifies the complex reaction system and effectively reflects the main thermodynamic characteristics of red mud sintering.

2CaO + SiO₂ + Al₂O₃ = Ca₂Al₂SiO₇

(1)

CaO + 2Al₂O₃ = CaAl₄O₇

(2)

CaO + Fe₂O₃ = CaFe₂O₄

(3)

Using the chemical composition of red mud and the above reaction equations, the Gibbs free energy of each reaction at different temperatures was calculated using FactSage software. The calculated results are presented in Figure 6.

As shown in Figure 6, the Gibbs free energy of Equation (1) is significantly lower than that of Equations (2) and (3) at the same temperature. Therefore, Ca₂Al₂SiO₇ is preferentially formed compared with CaFe₂O₄ during the sintering process.

A simple comparison of ΔG among the three standard reactions is insufficient to verify the preferential formation of Ca₂Al₂SiO₇ during the actual sintering process. Considering the complexity of the multi-component system, it can only be concluded that its formation possesses a relatively strong thermodynamic tendency.

This explains why calcium aluminosilicate becomes the dominant component in the liquid phase. In addition, the relative amounts of the three reaction products are mainly determined by the chemical composition of the system rather than by temperature.

4.1.3. Influence of CaO/Al₂O₃ Ratio on Phase Formation in Sintered Ore

The formation amounts of Ca₂Al₂SiO₇, CaAl₄O₇, and CaFe₂O₄ were simulated for a system containing 100 g of red mud at 1000 °C with the addition of different amounts of CaO, corresponding to various CaO/Al₂O₃ ratios. The simulation results are presented in Figure 7. The results show that as the CaO/Al₂O₃ ratio increased, the formation amount of Ca₂Al₂SiO₇ increased significantly, while the formation of CaAl₄O₇ slightly decreased, and the amount of CaFe₂O₄ remained nearly unchanged. This indicates that the formation of Ca₂Al₂SiO₇ is mainly controlled by the amount of added CaO. With increasing CaO addition, Al₂O₃ preferentially reacted with CaO and SiO₂ to form calcium aluminosilicate and calcium aluminate, thereby suppressing the formation of calcium ferrite. In addition, in the presence of CaO and SiO₂, low-melting eutectic mixtures were formed, which reduced the melting temperature of the system. As a result, the slag system shifted from a high-melting fayalite-based system to a lower-melting silicate-based system.

This also explains why the initial liquid phase formation temperature decreases with increasing basicity. In conventional sintering, the basicity is typically around 1.8, and the Al₂O₃ content is relatively low, making it difficult to form low-melting eutectic compounds in the silicate system. In red mud sintering, the primary liquid phase is calcium aluminosilicate, whereas in conventional sintering the main liquid phase is calcium ferrite. This difference represents one of the most significant distinctions between the two sintering processes.

4.2. The Influence of Na₂O on Sintering

Red mud contains Na₂O, and the presence of Na can effectively increase the basicity of the system. Phase diagram simulations using FactSage software were carried out under different basicity and temperature conditions. The results show that, compared with red mud without Na₂O, the presence of Na₂O reduced the initial liquid phase formation temperature by approximately 50 °C.

In addition, simulations were conducted by adding different amounts of CaO to 100 g of red mud to determine the main Na-bearing phases formed under various basicity conditions. The calculated results are shown in Figure 6. In the solid phase, Na₂O reacted with calcium aluminates to form Na₂Ca₃Al₁₆O₂₈. The formation amounts of Na₂Ca₃Al₁₆O₂₈ under different basicity and temperature conditions are presented in Figure 8.

According to the simulation results, Na₂Ca₃Al₁₆O₂₈ began to form at approximately 300 °C, accounting for about 12 wt% of the total solid phase. This compound started to decompose at temperatures above 1050 °C, and the decomposition temperature varied with basicity. When the basicity was 2.6, the decomposition temperature of Na₂Ca₃Al₁₆O₂₈ was approximately 1150 °C, whereas when the basicity increased to 4.8, the decomposition temperature decreased to about 1050 °C. This indicates that the decomposition temperature of Na₂Ca₃Al₁₆O₂₈ decreases with increasing basicity.

The melting point of CaO·2Al₂O₃ is approximately 1600 °C, whereas that of Na₂Ca₃Al₁₆O₂₈ is around 1200 °C. As a basic oxide, Na₂O reacts with Al₂O₃ and CaO in calcium aluminates at high temperatures to form low-melting eutectic mixtures, which disrupt the original crystal lattice and consequently reduce the melting temperature. Furthermore, Na₂O can also form low-melting eutectic compounds with SiO₂, CaO, and Ca₂Al₂SiO₇, further lowering the liquid phase formation temperature during sintering, which can be reduced to approximately 900 °C.

This mechanism explains why the liquid phase formation temperature differs by about 50 °C between red mud with and without Na₂O. Moreover, once the liquid phase forms, low-melting compounds such as NaAlO₂ and NaFeO₂ are generated, which further reduce the overall sintering temperature.

From the simulation results, it can be observed that the decomposition temperature of Na₂Ca₃Al₁₆O₂₈ decreases with increasing basicity. When the basicity is 2.6, the decomposition temperature is approximately 1150 °C, while at a basicity of 4.8, it decreases to about 1050 °C. The mineral phases of the sinter and the corresponding melting temperature curves were also simulated, and the results are shown in Figure 9. With increasing basicity and temperature, Na₂O gradually separates from calcium aluminates and enters the slag phase, where it reacts with iron and aluminum to form NaAlO₂ and NaFeO₂. At the initial stage, when the basicity is relatively low, the liquid phase formation temperature is high, and Na₂O remains mainly in the solid phase without entering the liquid phase. As the basicity increases, the liquid phase formation temperature decreases, and the amount of calcium aluminates in the solid phase increases. When the temperature exceeds approximately 1000 °C, Na₂O separates from calcium aluminates, enters the slag phase, and reacts with iron and aluminum to form NaAlO₂ and NaFeO₂, which further reduces the liquid phase formation temperature and promotes the sintering of red mud.

Based on these results, when sintering iron ores with high Al₂O₃ content, it is recommended to increase the basicity or add Na₂CO₃ in order to reduce the sintering temperature.

When the Na₂O content increases from 0.8% to 1.5%, the formation temperature of the initial liquid phase decreases by approximately 25–40 °C. This phenomenon can be attributed to the fact that Na₂O, as a strong basic oxide, is capable of disrupting the silicate network structure and promoting the formation of low-temperature eutectics, thereby lowering the temperature at which the liquid phase emerges. However, excessively high Na₂O content may lead to a reduction in sinter strength and the problem of cyclic enrichment of alkali metals. Therefore, it is necessary to control its content within a reasonable range.

4.3. The Influence of TiO₂ on Sintering

Thermodynamic simulation results indicate that TiO₂ reacts with CaO to form CaTiO₃, with an approximate yield of 6.7 g. Notably, the yield remains constant as basicity increases. When the basicity reaches 3.4, the formation of CaTiO₃ decreases to less than 5% of the final yield, a level that can be neglected in subsequent analysis.

4.4. Performance Analysis of Final Smelting Slag

According to the chemical composition of sinter, the iron content in pig iron was set at 95%, with a coke ratio of 600 kg/t and pulverized coal injection rate of 120 kg/t. The calculated content of Al₂O₃ in the final smelting slag exceeded 32%, accompanied by a melting temperature higher than 1475°C and a viscosity above 1 Pa·s, indicating that slag modification was necessary. After adding 17% CaO to the final slag, the melting temperature decreased to 1425°C, and the viscosity decreased to 0.551 Pa·s. The melting temperature and viscosity of the modified slag can meet the operational requirements of small blast furnaces.

5. Conclusions

(1)

At a sinter basicity above 4.8 and a ferrous iron mass fraction of approximately 12%, the resulting red mud sinter meets key performance specifications. The modified final slag exhibits favorable melting temperature and viscosity, while the as-prepared sinter demonstrates comprehensive physicochemical properties suitable for small blast furnace smelting. This study provides a new pathway for red mud resource utilization and a theoretical basis for high-alumina ore sintering, supporting the advancement of related metallurgical engineering technologies. During the red mud sintering process, calcium aluminosilicate phase was detected in the liquid phase, which is speculated to be one of the major constituent phases. This phase formed low-melting-point eutectic mixtures when coexisting with CaO and SiO₂. The formation amount of this phase is predominantly determined by the Ca/Al ratio and remains relatively independent of temperature variations. This insight enhances our understanding of the sintering mechanism and aids in optimizing the process parameters.

(2)

Na₂O contained in red mud reacted with aluminates to form new low-melting-point phases, which contributed to a reduction in the sintering temperature. In addition, once Na₂O entered the liquid phase, it further lowered the initial liquid phase formation temperature and improved the formation characteristics of the sintering liquid phase. Theoretical simulation results show that the influence of TiO₂ on the sintering reaction is relatively limited within the studied condition range. This discovery has significant implications for improving the efficiency and cost-effectiveness of the sintering process.